YAML and Bless to dump subset of object fails for me

[Rud1ger Sch1erz]

Hi there,

I'm fiddling around with YAML and try to write out a subset of a data
object.

The example of the YAML documentation works fine for me:

use YAML qw(Dump Bless);

use strict;

my $hash = {apple => 'good', banana => 'bad', cauliflower => 'ugly'};
print Dump $hash;
Bless($hash)->keys(['banana', 'apple']);
print Dump $hash;

gives me:
---
apple: good
banana: bad
cauliflower: ugly
---
banana: bad
apple: good

But when I try to use Blessing of keys to filter and sort the output
of an object reference, YAML seems to ignore my key subset.

<code example>
#!/usr/bin/perl -w
#

{
package DailyBuildInfoSet;

sub new {
my($proto ) = @_;
my $class = ref($proto) || $proto || DailyBuildInfoSet;
my $self = {};

bless ($self, $class);

return $self;
}

sub value {
my $self = shift;
my $k = shift;
if (@_) { $self->{$k} = shift }

return $self->{$k};
}

1;
}

use YAML qw(Dump Bless);

use strict;

my $db = DailyBuildInfoSet->new();
$db->value( "DOB", '20071114' );
$db->value( "HMI_CL", '1113617' );
$db->value( "TC_CL", '1111316' );

print Dump $db;

Bless($db)->keys(['HMI_CL', 'TC_CL']);
print Dump $db;

</code example>

Output of this example is:

$ t3.Al
--- !!perl/hashailyBuildInfoSet
DOB: 20071114
HMI_CL: 1113617
TC_CL: 1111316
--- !!perl/hashailyBuildInfoSet
DOB: 20071114
HMI_CL: 1113617
TC_CL: 1111316

On the latter, I would expect (and like to have):

--- !!perl/hashailyBuildInfoSet
HMI_CL: 1113617
TC_CL: 1111316

BTW, perl -v gives me:
This is perl, v5.8.8 built for cygwin-thread-multi-64int
(with 8 registered patches, see perl -V for more detail)

Could some kind soul point me to the reason, why YAML seems to ignore
my key blessing?

Cheers,
Rudiger

 

[jl_post]

I'm fiddling around with YAML and try to write out a subset of a data
object.

But when I try to use Blessing of keys to filter and sort the output
of an object reference, YAML seems to ignore my key subset.

<code example>
#!/usr/bin/perl -w
#

{
package DailyBuildInfoSet;

sub new {
my($proto ) = @_;
my $class = ref($proto) || $proto || DailyBuildInfoSet;
my $self = {};

bless ($self, $class);

return $self;
}

sub value {
my $self = shift;
my $k = shift;
if (@_) { $self->{$k} = shift }

return $self->{$k};
}

1;

}

use YAML qw(Dump Bless);

use strict;

my $db = DailyBuildInfoSet->new();
$db->value( "DOB", '20071114' );
$db->value( "HMI_CL", '1113617' );
$db->value( "TC_CL", '1111316' );

print Dump $db;

Bless($db)->keys(['HMI_CL', 'TC_CL']);
print Dump $db;

</code example>

Output of this example is:

--- !!perl/hashailyBuildInfoSet
DOB: 20071114
HMI_CL: 1113617
TC_CL: 1111316
--- !!perl/hashailyBuildInfoSet
DOB: 20071114
HMI_CL: 1113617
TC_CL: 1111316

On the latter, I would expect (and like to have):

--- !!perl/hashailyBuildInfoSet
HMI_CL: 1113617
TC_CL: 1111316

Could some kind soul point me to the reason, why YAML seems to ignore
my key blessing?

Dear Rudiger,

Hmmm... that is strange.

I've been tinkering around with your program, and I've discovered
something a little strange.

Try replacing these four lines of your code:

my $db = DailyBuildInfoSet->new();
$db->value( "DOB", '20071114' );
$db->value( "HMI_CL", '1113617' );
$db->value( "TC_CL", '1111316' );

with the following four lines:

$db = { DOB => '20071114',
HMI_CL => '1113617',
TC_CL => '1111316' };
bless $db, "DailyBuildInfoSet";

These new four lines do pretty much the same as the old four lines.
And as you'd expect, the output from YAML:ump() is exactly the same
(which includes the "DOB" line).

(In fact, you can change the bless() line to just "bless $db;" and the
problem still persists.)

But try this: Remove (or comment-out) the bless() line (the fourth
line of the new code) and run the script again. When I run the
script, the YAML:ump() output no longer includes the "DOB" line.

Apparently the fact that the $db object is bless()ed (with Perl's
bless(), not YAML's Bless()) makes all the difference. I don't know
why bless()ing a reference would make YAML::Bless() and/or
YAML:ump() work differently than what you'd expect -- it just does.

(The "perldoc YAML" documentation says that Bless() associates a
"normal Perl node." Maybe that means that a bless()ed object is not a
normal Perl node. Or maybe not and it's just a bug in the YAML
module.)

Sorry I can't help you more, but at least we know that the
difference is happening because the $db object was bless()ed.

I hope this helped (if even in a small way), Rudiger.

-- Jean-Luc Romano